Adding and updating key-value pairs in Python - Step-by-Step Execution

Concept Flow - Adding and updating key-value pairs

Start with dictionary

↓

Add new key-value pair?

Yes→Add key and value

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Update existing key's value

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Dictionary updated

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End

Start with a dictionary, then add a new key-value pair or update an existing key's value, resulting in an updated dictionary.

Execution Sample

Python

my_dict = {'a': 1, 'b': 2}
my_dict['c'] = 3
my_dict['a'] = 4
print(my_dict)

This code adds a new key 'c' with value 3 and updates the value of existing key 'a' to 4.

Execution Table

Step	Action	Dictionary State	Output
1	Initialize dictionary	{'a': 1, 'b': 2}
2	Add key 'c' with value 3	{'a': 1, 'b': 2, 'c': 3}
3	Update key 'a' to value 4	{'a': 4, 'b': 2, 'c': 3}
4	Print dictionary	{'a': 4, 'b': 2, 'c': 3}	{'a': 4, 'b': 2, 'c': 3}

💡 All steps executed; dictionary updated and printed.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
my_dict	{'a': 1, 'b': 2}	{'a': 1, 'b': 2, 'c': 3}	{'a': 4, 'b': 2, 'c': 3}	{'a': 4, 'b': 2, 'c': 3}

Key Moments - 2 Insights

Why does updating an existing key overwrite its old value instead of adding a new key?

Because dictionary keys are unique, assigning a value to an existing key replaces the old value, as shown in step 3 of the execution_table.

What happens if we add a key that wasn't in the dictionary before?

A new key-value pair is added to the dictionary, increasing its size, as seen in step 2 where key 'c' is added.

Visual Quiz - 3 Questions

Look at the execution_table, what is the dictionary state after step 2?

A {'a': 4, 'b': 2, 'c': 3}

B {'a': 1, 'b': 2, 'c': 3}

C {'a': 1, 'b': 2}

D {'a': 1, 'b': 2, 'a': 4}

💡 Hint

Check the 'Dictionary State' column in row for step 2 in execution_table.

At which step does the value of key 'a' change?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at the 'Action' and 'Dictionary State' columns in execution_table for step 3.

If we add a new key 'd' with value 5 after step 3, what will be the final dictionary?

A {'a': 4, 'b': 2, 'd': 5}

B {'a': 1, 'b': 2, 'c': 3, 'd': 5}

C {'a': 4, 'b': 2, 'c': 3, 'd': 5}

D {'a': 4, 'b': 2, 'c': 3}

💡 Hint

Adding a new key adds it to the dictionary without removing existing keys, see step 2 for similar action.

Concept Snapshot

Adding and updating key-value pairs in a dictionary:
- Use dict[key] = value
- If key is new, pair is added
- If key exists, value is updated
- Keys are unique
- Dictionary changes immediately

Full Transcript

We start with a dictionary containing keys 'a' and 'b'. Then we add a new key 'c' with value 3, which adds a new pair. Next, we update the value of existing key 'a' to 4, replacing the old value. Finally, we print the dictionary showing all changes. This shows how to add new pairs and update existing ones in Python dictionaries.